This invention relates generally to circuits for compensating for the temperature characteristics of primary elements for measuring various physical quantities.
The temperature characteristics of primary elements usually require compensation in order to preserve the accuracy of the measurement over a wide range of ambient temperatures. An example of such a primary element is the differential pressure cell which is used in the measurement of fluid flows. A representative differential pressure cell is shown and described in U.S. Pat. No. 4,257,274. That particular transducer has a silicon diaphragm anodically bonded between two glass plates to form a pair of capacitors on opposing sides of the diaphragm. The capacity exhibited by these capacitors varies in an opposite sense with changes in differential pressure. Typically these transducers are utilized in a measuring circuit which serves to provide a signal which varies in proportion to the change in capacity relative to the total capacity, .DELTA.C/.SIGMA.C.
The temperature changes which occur in differential pressure cells as well as other primary elements due to changes in ambient temperature conditions under which the elements operate, causes a variation in the change in the output voltage of the cell with changes in the quantity being measured. Such changes are usually referred to as a change in "span". For example, in a differential pressure cell, the range over which the differential pressure cell output changes with a particular change in differential pressure will vary with temperature. Therefore the "span" will require adjustment or compensation.
The temperature coefficient of the primary element also usually causes a change in the offset voltage of the element. Thus, for example, a differential pressure cell will normally exhibit a particular output voltage at a reference temperature for a certain value of differential pressure. However, that offset voltage will vary with temperature and thereby create a need for compensation. These offsets are frequently referred to as the "zero" since they usually represent the output at the lower end of a range of measurements. The range then represents the "span".
Prior art circuits for providing temperature compensation of span have utilized thermistors placed in the voltage supply to the bridge circuit of the differential pressure cell. This provided a resulting change in the excitation voltage across the transducer to compensate for any changes in the output of the transducers which occurred with changes in temperature. The thermistor maintained the excitation across the cell at the desired value by varying its resistance with temperature.
U.S. Pat. No. 4,326,171 describes a temperature compensating circuit wherein a thermistor is used to maintain a constant excitation voltage in the manner described above. Circuits of the type shown in this patent tend to be expensive because of the need for using precision components and using complex trimming procedures in order to cause the circuit in which the thermistor is inserted to provide the desired temperature characteristics.
The electronic measuring circuit associated with the primary element may itself exhibit a significant "zero" shift with temperature.